The Center Is Ancient

Most astronomers think the Milky Way’s suburbs are its oldest part—they may be wrong.

Ask most astronomers where to find the oldest stars in the galaxy and they’ll tell you to look at globular clusters, dense knots of stars that hover above and below the plane of the Milky Way. But Young-Wook Lee of Yonsei University Observatory in Seoul, South Korea, says most astronomers have been looking in the wrong place. Some stars in globular clusters may be 15 billion years old, he says, but the great bulge at the center of the Milky Way--a younger part of the galaxy, according to conventional wisdom--actually holds stars that are 1 or 2 billion years older. That difference matters, because the age of the oldest stars sets a lower limit on the age of the universe. What’s more, having the oldest stars at the center of the galaxy would contradict the conventional model of how our galaxy formed.

Most globular clusters are judged to be ancient because the stars in them suffer from a sort of cosmic anemia--they contain very few metals. (For astronomers, anything heavier than helium is a metal.) The Big Bang, cosmologists believe, created only the very lightest elements: hydrogen, helium, and perhaps some lithium. All the others were made by fusion in the cores of stars, or in the fantastic heat and pressure of a supernova explosion. They were then scattered into space, where they became the raw material for new stars. Thus many of the heavier elements found in typical later-born stars like the sun were actually forged by stars of an earlier generation. Stars in globular clusters, however, have less than a tenth the metal abundance of the sun. That suggests they had few or no stellar forebears but were instead among the first stars to light up the galaxy.

Lee doesn’t dispute that globular cluster stars are old--only that they are the oldest. He has developed a different way of finding the oldest stars. It involves counting the number of certifiably geriatric stars in a group.

The idea is a little complex, so an analogy may help. Imagine you wanted to measure the age of a forest. (Assume it’s no older than an individual tree could be.) One way would be to count the number of trees that have clearly been around for a while--say, ones that had grown taller than 100 feet. The more trees you find that have had time to reach that height, the older the forest would be. But before you could conclude that your forest is older than one in the next state, you’d have to allow for complicating factors that affect a tree’s growth rate--soil conditions, for instance.

Lee compared groups of stars from two different regions of the galaxy: the central bulge and the roughly spherical, globular-cluster- studded halo that surrounds the Milky Way disk. His equivalent of 100-foot trees was a class of stars called RR Lyraes: bright, variable, and low-mass stars named for the constellation Lyra, where astronomers first spotted one more than a century ago. Unlike the sun, which generates energy by fusing hydrogen atoms into helium, RR Lyrae stars have already used up all the hydrogen in their core and are fusing helium into carbon instead. Thus they must be relatively old.

But as in the case of the forest, one cannot conclude that one group of stars is older than another solely because it contains more RR Lyraes. There are complicating factors--factors that affect how fast a star evolves into an RR Lyrae. The most important one, confusingly enough, is the same thing that astronomers have traditionally used as an indicator of a star’s age: its metal content. If two stars are born at the same time, the one that for whatever reason happens to be endowed at birth with more metals will take longer to evolve into an RR Lyrae. And a metal-rich region of the galaxy will take longer than a metal-poor one to produce a lot of RR Lyraes--just as a forest growing on poor soil will take longer to produce a lot of 100-foot trees.

The bulge of the Milky Way is more metal-rich than the halo. But when Lee examined the proportion of stars that had evolved into RR Lyraes in the two regions, he did not find a higher proportion in the halo, as one would expect: he found a higher proportion of RR Lyraes in the bulge. The only way the bulge could have produced so many 100-foot trees in such infertile soil, Lee concluded, was to have been working at it longer than the halo. He calculated that the oldest stars in the galactic center had to be at least one billion years older than the oldest stars in globular clusters to allow enough time for so many RR Lyraes to evolve there.

If Lee is right, his results are significant for at least two reasons. The first is simply that the results contradict the basic assumption that the metal-poorest stars are necessarily the oldest. If you think of a closed-box model of the galaxy, then the metal-poor stars must be older than the metal-rich stars, says Lee. But the galaxy is not a closed box. Supernova explosions in older, nearby galaxies, says Lee, may have fertilized the nascent Milky Way--and in particular its center--with some of the heavy elements we now find here.

Alternatively, or perhaps additionally, short-lived members of an older generation of stars within the Milky Way itself may have enriched the galactic bulge with metals. That lost generation, Lee thinks, would have been more likely to pop up and pop off in the center of the galaxy, where there was lots of gas to make stars with.

That leads to the second and more sweeping implication of Lee’s results: what they say about how our galaxy formed. In the standard picture, the globular clusters in the halo limn the shape of a single vast protogalactic cloud, which gave birth to the globular clusters as it collapsed to form the galaxy. But if the oldest stars are at the center of the galaxy rather than in the halo, then something is wrong with the standard picture.

Lee thinks the galaxy probably formed not from the cataclysmic collapse of one big gas cloud but from the mergers of many smaller ones. (This alternative theory is not new; in fact, Lee’s mentor, Yale astronomer Robert Zinn, helped develop it about 15 years ago.) Stars would have appeared first where the clouds collided, in what became the dense center of the primordial galaxy, and only later in the more tenuous halo. Basically the textbook picture of galaxy formation is kind of an outside-in picture, says Lee. Mine is an inside-out picture. It’s exactly the opposite view.